Invasion of Poisonous Asian toad in Madagascar is a greater threat to biodiversity than previously thought

Invasive species are among the greatest threats to the world’s biodiversity, and amphibians rank among the most devastating vertebrate invaders. That’s why the discovery of the Asian toad (Duttaphrynus melanostictus) in Madagascar’s second largest city and main port of Toamasina in March 2014 so alarmed conservationists.

Should the newly introduced toad spread across the island nation, it could do extreme harm to Madagascar’s extraordinary, but already threatened biodiversity. An astounding 84 percent of the country’s fauna is endemic to the island, with many species having little defense against invaders.

A new study published in mongabay.com’s open-access journal Tropical Conservation Science indicates that the Asian toad may have arrived in Madagascar by 2010 or earlier, and went undetected until 2014. Investigators have now found the toad to be widespread and numerous in a large swath of Toamasina, to the south and west of the city center, an area of at least 108 square kilometers (41.7 square miles).

D. melanostictus is a pernicious invader. In its native Asia, it inhabits a wide range of environments, including forests, agricultural land and urban areas, and is found at altitudes up to 2,000 meters (6,562 feet) – which means it has the potential to spread through an equally wide range of Madagascar’s sensitive ecosystems. When the toad’s Asian ecological niche was modeled, it was revealed that Madagascar’s climate, especially its east coast, is ideal for the toad’s spread. The Asian toad is also highly fecund, reproducing most of the year. It is also poisonous and deadly, even to humans, posing a health risk. And it is voracious: one study found invertebrates from more than 20 families within the stomach contents of a single Asian toad.

The Asian toad in Toamasina showing its distinct appearance and size which is unique and unlike any other amphibian species in Madagascar. Photo courtesy Maya Moore.

The discovery of D. melanostictus in Madagascar in 2014 raised urgent scientific issues. Most importantly: “When an invasive species is detected, determining its distribution is crucial in order to predict the rate at which a species will expand its range and to inform eradication or control strategies,” write the scientists. “These data, however, are often limited to randomly collected observations”

Researchers Maya Moore and Jean Francois Solofo Niaina Fidy of the Madagascar Fauna and Flora Group, and Devin Edmonds of the Association Mitsinjo, worked to systematically fill in the missing current range data. They conducted 516 social interviews (asking local residents when they first observed the toad, and whether they considered many to be present), and also performed 120 visual encounter surveys in Toamasina between April and November 2014.

More than 350 of the residents surveyed reported seeing the toads, mostly in 2014, but a few spotted them as early as 2010. The visual surveys, conducted at night, located toads at 48 sites, including urban environments, rural villages, agricultural land and rice paddies, mixed Eucalyptus forests, secondary vegetation and grassy areas.

The scientists note that no toads were found in or around the port, their most likely point of entry into Madagascar. “This suggests that the toad could have arrived at the port inside a shipping container, then was moved elsewhere inside the container to become established farther south,” the investigators write. “Social surveys indicate that the toad may have already been present for some years and potentially introduced prior to 2010, with the site of its introduction likely south of Toamasina near National Route 2 and the Ambatovy plant.”

Delineating the current edge of the toad’s range is crucial, say the researchers, if its population is to be controlled, or possibly eradicated. “[I]t is vital that more sensitive survey methods be immediately tested and employed, such as investigating with the use of tracking tunnels, acoustic monitoring and environmental DNA,” they write.

More surveys using standard methodology are urgently needed, and those surveys need to include private property areas such as the Ambatovy mining operation, from which the scientists were refused access, but around which Asian toads have well-established populations. Ambatovy staff have assisted by conducting their own surveys and found the toad on site. “[I]f an eradication effort is to be successful it will be essential to avoid splitting work between staff from the eradication program and private entities who may not follow the same protocol or have background in invasive species removal,” write the authors.

There are also concerns that the toad may have already been accidentally transported well beyond its initial entry point, as the animals can easily conceal themselves in cargo or trash being transported by truck or rail. The toad has been reported along the Salambona River, more than 220 kilometers (136.7 miles) south of Toamasina, though these sightings are not confirmed.

The task of eradicating the Asian toad from Madagascar is a formidable one. To be successful “every individual toad must be destroyed at a rate faster than their ability to breed and recruit over an area now greater than 100 square kilometers [38.6 square miles].” Such an extreme eradication effort will be extraordinarily difficult to organize considering Madagascar’s limited resources. One major concern: if biosecurity is not quickly improved and the toad is not eradicated, it could spread not only to Madagascar’s wilds, but be transported into other nations to threaten their biodiversity.

“[I]nvasive species have received far less attention in Madagascar than other threats facing the island’s unique biodiversity,” write the researchers. “The introduction of D. melanostictus and failure to discover it until it was already widespread throughout Toamasina underscores this point, highlighting the need for conservation organizations and the scientific community to improve efforts to identify and address the threats presented by invasive alien species. With this in mind, we see an opportunity for a coordinated eradication effort to lead to critical methodological and strategic advances for the removal of pest amphibians globally, whilst resulting in improved policies and biosecurity procedures for the country of Madagascar.”